This invention is directed to door locking devices and, more particularly, to electromagnetically controlled door locks that are actuatable from a remote location. Such locks, known generally as "electric strikes," are commonly employed for controlling access to hotel, office and apartment buildings from various remote locations throughout the building.
A common denominator of all electric strikes is a latch bolt keeper or strike tongue that is movable from an operative latch bolt securing position to an inoperative latch bolt releasing position. A retractable stop member engages the strike tongue and holds it in its latch bolt securing position. Hence upon retraction of the stop member, which is effected by a remotely controlled solenoid, the strike tongue can be moved out of its operative position by opening force applied to the door.
In prior electric strikes the manner in which the retractable stop member held the strike tongue in its operative position greatly resisted retraction of the stop member and often prevented it if opening force was applied to the door simultaneously with actuation of the solenoid to unlock the door. As a result, the door would not open. This unfortunate and obviously objectionable consequence resulted from the fact that the door opening force applied by the hopeful entrant and transmitted through the latch bolt and the movable strike tongue, to the stop member was often sufficient to override the solenoid-produced retracting force.
One solution to this problem of a thus jammed or restrained strike tongue is to utilize a solenoid having sufficient capacity to generate enough power to overcome the restraining force exerted against the stop member in consequence of door opening force applied simultaneously with energization of the solenoid. But, a solenoid strong enough to do this is expensive and costly to operate.
Another disadvantage of some of the electric strikes heretofore available is the ease with which they could be defeated by insertion of a thin blade-like instrument between the movable strike tongue and its stop member, or by striking the door frame at the location of the strike tongue with a sharp blow to jar the solenoid and thereby cause it to retract the stop member.
A solution to the aforesaid problems heretofore proposed was to maintain the stop member in its projected strike tongue securing position by a continuously energized solenoid powerful enough to prevent release of the strike tongue by the insertion of a blade-like instrument between the movable strike tongue and its stop member, by striking the door frame with a sharp blow or by the application of relatively strong lock defeating forces to the door. Obviously, that manner of assuring the desired security demands an expensive solenoid requiring continuously applied power. Moreover, locks having solenoids in a continuously energized state tend to be noisy and, of course would be defeated by a power interruption.